1. Field of the Invention
The invention relates to extrusion apparatus for producing films or sheets and more particularly to extrusion apparatus capable of coextruding two or more flat layers of materials laminatable to each other and/or other substrates.
2. Prior Art
Apparatus for the coextrusion of flat multi-layer films of at least certain thermo-extrudable material is known. Certain of such apparatus includes separate manifolds feeding individual melts to a common passageway so that contiguous boundaries between adjoining melts are formed and the resulting melt stream exists from the apparatus as a composite film. However, in such systems complex adjustments are necessary to achieve individual melt thickness control and only materials having similar resin rheology are usable. Furthermore, individual temperature control for each melt in such systems is not readily attained, especially during flow over the common passageway. Other known apparatuses utilize a single manifold for receiving a plurality of melts and a special adapter within the manifold lines up the melt stream in a desired sequence. In such systems, the resins utilized must have about equal apparent melt viscosities in order to attain uniformity in thickness ratios and individual melt temperature control is not readily attainable because of the contiguous flow streams of the various melts. Yet other apparatuses which are previously known produce composite blown film wherein different melts are combined inside or outside a die. Blown films require additional processing, that is, collapsing of the bubble, slitting, folding, etc. and are thus considered relatively undesirable for the production of sheet products. Further, composite blown films do not exhibit sufficient bonding between the individual layers. Blown film dies wherein the different melts combined inside the die do not allow individual melt temperature and/or thickness control. Known blown film dies wherein the different melts combine outside of the die are incapable of adjusting the individual layer thicknesses or of regulating the temperature of each melt. All of the various known coextrusion flat film apparatuses allow edge buildup on the extruded layers, which is highly undesirable and comprises waste.
It is an object of the invention to provide an improved coextrusion apparatus for flat film which includes precise temperature control for each melt, and precise control for each extruded layer, with permissibly the avoidance of edge buildup.